flex fails to build on Windows

Submitted by Sebastian Dröge (slomo)

Link to original bug (#745804)

Description

This happens because
a) flex requires POSIX regex (i.e. regex.h), for that I already packaged the mingw standalone version of it. Needs to be integrated into the recipe for Windows but that's simple and works
b) flex uses various other POSIX API not available on Windows, like fork() and wait(). There must be compatibility implementations available somewhere as mingw ships flex binaries but I didn't find them yet and also not the build steps used to build the mingw flex binaries

Must be fixed before we can do a new release.